The present invention relates generally to photographic equipment and particularly to automatic exposure control systems in a camera.
Automatic exposure control is known in a photographic camera with exchangeable lenses where each lens is equipped with an automatically adjusting diaphragm. A control device determines the magnitude of the working aperture by comparing a signal potential which is proportional to a shutter timing value and serves as the reference with a signal potential which is proportional to the luminance of the scene, the sensitivity of the film and the existing diaphragm aperture of the taking lens used. The control device passes the signal resulting from said comparison to a timing unit. Processes and devices of this type are intended to control the exposure in a photographic camera so that following the selection of a shutter speed value the diaphragm of the lens being used adjusts automatically as a function of the scene luminance and film sensitivity to an aperture yielding the correct exposure.
In West German Offenlegungsschrift No. 24 33 894 equivalent to U.S. Pat. No. 3,964,073 an exposure control device for a photographic camera is disclosed wherein an exposure measuring circuit initially produces a signal proportional to the luminance of the scene, film sensitivity and the initial aperture of the lens used. The signal is compared with the signal indicative of the shutter speed value preselected on the camera. The difference between the two potentials is an indication of the setting of the lens diaphragm aperture necessary to produce a correct exposure. Due to the movement of the lens diaphragm, released by the triggering of the camera, the signal at the output of the exposure measuring circuit varies continuously. When the potentials of the signals of the exposure measuring circuit and the shutter speed value are equal, a comparator activates a magnet and stops the closing of the diaphragm at the working aperture.
Faults inherent in the diaphragm arresting device caused by certain time constants as there are the dropping off of a magnet, the falling in of latches etc, for instance operate to permit the closing of the diaphragm beyond the set value intended and are detrimental to the exact shutter speed desired for exposure of the film.